-
Morro Bay Total Maximum Daily Load for Sediment (including
Chorro Creek, Los Osos
Creek and the Morro Bay Estuary)
State of California
Central Coast Regional Water Quality Control Board Prepared on
April 24, 2002
Staff contact information: Katie McNeill 81 Higuera Street San
Luis Obispo, CA 93401 (805) 549-3336 [email protected]
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
i
Morro Bay, Chorro Creek and Los Osos Creek
Total Maximum Daily Load For Sediment
Table of Contents
1 .
Introduction..............................................................................................................................................
1 1.1 Documents
Used..........................................................................................................................
1
2 . Problem Statement
...................................................................................................................................
2 2.1 Overview/General Problem
.........................................................................................................
2 2.2 Water Quality
Standards..............................................................................................................
2
2.2.1 Beneficial
Uses........................................................................................................................
2 2.2.2 Water Quality Objectives
........................................................................................................
4
2.3 Description of Morro Bay and the Morro Bay Watershed
.......................................................... 4 2.4
Stream Discharge into Morro
Bay...............................................................................................
6 2.5 Sedimentation in Morro Bay
.......................................................................................................
6
2.5.1 Background
Erosion................................................................................................................
6 2.5.2 Estimates of Sediment
Loading...............................................................................................
7 2.5.3 Bathymetry and Sediment Flushing
........................................................................................
7 2.5.4 Sedimentation in Chorro and Los Osos Creek Watersheds
.................................................. 11 2.5.5
Highway-41 Fire of 1994
......................................................................................................
12
2.6 Impacts to Beneficial Uses
........................................................................................................
12 2.6.1 Fish and Wildlife (RARE, MIGR, SPWN,
WILD)...............................................................
12 2.6.2 Freshwater Habitat (COLD,
WARM)...................................................................................
14 2.6.3 Estuarine and Marine Habitat (EST, MAR,
BIOL)...............................................................
16 2.6.4 Summary of Biological Beneficial Use Impacts
...................................................................
17 2.6.5 Water Contact and Non-Contact Recreation, Navigation
(REC1, REC2, NAV) ................. 18 2.6.6 Shellfish Harvesting,
Aquaculture, and Commercial and Sport Fishing (SHELL, AQUA, COMM)
..............................................................................................................................................
19 2.6.7 Industrial
(IND).....................................................................................................................
19 2.6.8 Municipal, Agricultural Supply, Freshwater Replenishment
(MUN, AGR, FRESH) .......... 19
3 . Source Analysis
.....................................................................................................................................
20 3.1 General
Overview......................................................................................................................
20 3.2 Methods
.....................................................................................................................................
20
3.2.1 Base Load Estimation
Methods.............................................................................................
20 3.2.2 Methods to Assign Loads to Erosion
Types..........................................................................
24 3.2.3 Method to Assign Loads to Land Use Types
........................................................................
25
3.3 Relative
Contributions...............................................................................................................
29 3.3.1 Total Loading from Subwatersheds
......................................................................................
29 3.3.2 Loading by Erosion Source Category
...................................................................................
30 3.3.3 Loading from Sheet and Rill Erosion by Land Use
.............................................................. 31
3.3.4
Mines.....................................................................................................................................
32
4 . Numeric
Targets.....................................................................................................................................
33 4.1 Chorro Creek and Los Osos Creek Numeric Targets
................................................................
34
4.1.1 Streambed Sediment
Targets.................................................................................................
34 4.1.2 Creek Water Column Target
.................................................................................................
39
4.2 Morro Bay and Estuary Target
..................................................................................................
39
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
ii
4.2.1 Tidal Prism Volume
..............................................................................................................
39 5 . Linkage Analysis
...................................................................................................................................
42 6 . Total Maximum Load and Load Allocations
.........................................................................................
44
6.1 TMDL Calculation
....................................................................................................................
44 6.2 Margin of
Safety........................................................................................................................
46 6.3 Temporal
Considerations...........................................................................................................
46
7 . Public
Participation................................................................................................................................
48 8 . Implementation Plan
..............................................................................................................................
49
8.1 Introduction
...............................................................................................................................
49 8.1.1 Watershed-Wide Implementation
.........................................................................................
49
8.2 Existing Sediment Control Programs
........................................................................................
50 8.2.1 Morro Bay Comprehensive Conservation Management Program
........................................ 50 8.2.2 Morro Bay
Watershed Enhancement
Program......................................................................
50 8.2.3 Farm Bureau Watershed
Program.........................................................................................
51
8.3 Implementation Actions to Reduce Sediment
...........................................................................
52 8.3.1 Sediment Reduction
Activities..............................................................................................
52 8.3.2 Trackable Implementation Actions
.......................................................................................
55
8.4 Regulatory Mechanism by which TMDL Implementation is
Assured...................................... 59 8.4.1 Regional
Board Authority to Require Implementation
......................................................... 59 8.4.2
Regulatory Control Measures to Reduce Sedimentation
...................................................... 60
8.5 Schedule of
Compliance............................................................................................................
62 8.6 Demonstrating Compliance
.......................................................................................................
65
8.6.1 Measures of Success
.............................................................................................................
65 8.6.2 Failure
Scenarios...................................................................................................................
66 8.6.3 Compliance Assurance and Enforcement
.............................................................................
67
8.7
Cost............................................................................................................................................
67 8.7.1 Cost of Trackable Implementation
Actions...........................................................................
68 8.7.2 Cost of Erosion Control
BMPs..............................................................................................
68 8.7.3 Total Estimate of Implementation Costs
...............................................................................
69 8.7.4 Cost of
Monitoring................................................................................................................
69
9 . Monitoring Plan
.....................................................................................................................................
71 9.1 Coordination
..............................................................................................................................
71 9.2 Monitoring Numeric Targets
.....................................................................................................
71
9.2.1 Streambed Sediment Target Monitoring
...............................................................................
72 9.2.2 Turbidity Target Monitoring
.................................................................................................
73 9.2.3 Tidal Prism Volume Target
Monitoring................................................................................
73
9.3 Monitoring Implementation Actions
.........................................................................................
73 9.4 Data
Management......................................................................................................................
73
10 . List of References
................................................................................................................................
75 APPENDIX: Comprehensive Conservation Management Program Project
Descriptions (Trackable Implementation Actions)
............................................................................................................................
79
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
iii
List of Tables
Table 1. Identified Uses of Inland Surface and Coastal Waters of
the Morro Bay Watershed..................... 2 Table 2. Estimated
Discharge for Points along Chorro and Los Osos Creeks for Events of
Different
Magnitudes............................................................................................................................................
6 Table 3. Hyposometric (Area vs. Depth) Data Summary for Morro
Bay, 1884 to 1998............................. 8 Table 3.
Hyposometric (Area vs. Depth) Data Summary for Morro Bay, 1884 to
1998............................. 9 Table 4. Adjusted Volume-Depth
Relationship for Morro Bay, 1884-1998.
............................................... 9 Table 5. Area
Burned During Highway-41 Fire.
......................................................................................
12 Table 6. Special Status Species Dependent on Morro Bay Estuary
and Watershed................................... 13 Table 7. Areal
Extent of Estuarine Habitat in Acres Reported by Various
Investigators........................... 16 Table 8. Summary of
impacts to habitats associated with sedimentation in Morro Bay.
........................... 18 Table 9. Subwatersheds of Morro Bay
Watershed......................................................................................
22 Table 10. Erosion Categories and Percent Contribution in Morro
Bay Watershed. ................................... 24 Table 11.
Land Uses (acres) within Chorro and Los Osos Creek Watersheds.
.......................................... 26 Table 12. SCS
Estimates of Sheet and Rill Sediment Load for Land Uses in Morro
Bay Watershed. ...... 27 Table 13. Conversion Factors used to
convert SCS’s estimates to Tetra Tech’s estimates.
....................... 27 Table 14. Adjusted Load from Sheet and
Rill Erosion on Land Uses in Chorro and Los Osos Creek
Watersheds.
.........................................................................................................................................
27 Table 15. Sediment Yield from Sheet and Rill Erosion by Land
Use. ....................................................... 28
Table 16. Unadjusted Sediment Load (tons/year) from Sheet and Rill
Erosion on Land Uses within
Subwatersheds.....................................................................................................................................
28 Table 17. Adjustment factors for subwatershed loads from sheet
and rill erosion. .................................... 29 Table 18:
Event-based and Annual Average
Loadings...............................................................................
30 Table 19. Estimated Sediment Load (tons/year) by Erosion
Category to Morro Bay. ............................... 31 Table 20.
Adjusted values for Sediment Load from Sheet and Rill Erosion on
Land Uses in Morro Bay
Watershed.
..........................................................................................................................................
32 Table 21. Numeric Targets
.........................................................................................................................
34 Table 23. Load Allocations for Four Erosion Categories in Morro
Bay Watershed .................................. 45 Table 24. Load
Allocations for Land Uses in Morro Bay Watershed (Sheet and Rill
only) ...................... 45 Table 25. Typical Sediment
Reduction Rates from BMPs
.........................................................................
53 Table 26. Current Sediment Yield, Typical BMP Reduction Rates,
and the Resulting Loading by Erosion
Category in Chorro and Los Osos
Creeks...........................................................................................
54 Table 27. Typical BMP Reduction Rates, and the Resulting Loading
Rate from Sheet and Rill Erosion by
Land Use Category in Chorro and Los Osos Creeks.
.........................................................................
54 Table 28. Sediment Loads to be Collected by Sediment Capture
Projects to Achieve the TMDL. ........... 55 Table 29. Trackable
Implementation
Actions.............................................................................................
58 Table 30. Implementation Compliance Schedule for Sediment TMDL
for Morro Bay ............................. 63 Table 31. Example
Annual Costs for On-Site
BMPs..................................................................................
69 Table 32. Estimate of Annualized Cost to CCRWQCB for Monitoring
TMDL Implementation .............. 70 Table 33. Monitoring Plan
..........................................................................................................................
72
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
iv
List of Figures Figure 1. The Setting of Morro
Bay……………………………………………………………….……… 6 Figure 2. Map Showing Current
Bathymetry of Morro Bay……………………………………….………9 Figure 3: Subwatersheds
of Morro Bay.
.....................................................................................................
21 Figure 4. Established TMDL Monitoring Sites.
.........................................................................................
36 Figure 5: Projected Tidal Prism
Volumes...................................................................................................
40 Figure 6: Tidal Prism Volume Target-Setting
............................................................................................
41
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
1
1. Introduction A Total Maximum Daily Load is the greatest
amount of a particular pollutant that a waterbody can receive
without exceeding the water quality objectives established to
protect the beneficial uses of that waterbody. The Federal Clean
Water Act requires Total Maximum Daily Loads (TMDLs) for waters
that exceed water quality standards or objectives. The TMDL, which
can also be described as the loading capacity, is expressed by the
following formula: TMDL = ∑(Load from Point Sources)+∑(Load from
Nonpoint Sources)+∑(Load from Background / Natural Sources)+(Margin
of Safety) Chorro Creek, Los Osos Creek, Morro Bay, and the Morro
Bay Estuary1 are listed as waters impaired by
sedimentation/siltation, and are the subject of this TMDL. The
loading capacity for all of the waterbodies is addressed in one
TMDL, since the sources of sediment, nature of water quality
impairments, sources of water quality data, pollutant-loading
determinations, land uses, and water quality attainment strategies
are very similar. Furthermore, the waterbodies are all part of the
Morro Bay watershed and a watershed-wide approach was required to
develop an understanding of sedimentation in the Estuary, and to
address all controllable sources of sediment.
1.1 Documents Used A large volume of information concerning
Morro Bay’s natural resources was considered in preparing this
TMDL. Computer models constructed by Tetra Tech for the Morro Bay
National Estuary Program provided the basis of sediment loads
presented in the TMDL. Soil Conservation Service reports provided
the basis for the 50 percent load reduction identified as necessary
to protect beneficial uses. Among the numerous resources consulted,
the following reports were particularly valuable and relied upon
more than others:
• Sedimentation Processes in Morro Bay, California, Jeffrey
Haltiner, 1988. • Morro Bay Estuary Program Sediment Loading Study,
Tetra Tech, Inc. 1998.
• Morro Bay National Estuary Program (MBNEP) Watershed
Streamflow, Tetra Tech, Inc. 1998.
• Morro Bay National Estuary Program Habitat Characterization
and Assessment Study, Tetra
Tech, Inc. 1999.
• Erosion and Sediment Study Morro Bay Watershed, U.S. Dept of
Agriculture, Soil Conservation Service, 1989.
• Morro Bay Watershed Enhancement Plan, U.S. Dept of Agriculture
(USDA), Soil Conservation
Service (SCS), 1989.
1 In the Basin Plan, Morro Bay and the Morro Bay Estuary are
identified as “ocean” and “inland surface waters,”
respectively, for the purpose of identifying the beneficial uses
applied to these waters. The two names in fact refer to one
waterbody, which is recognized to begin landward of the breakwater
on the Pacific Ocean at Estero Bay.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
2
2. Problem Statement
2.1 Overview/General Problem Over time, all estuaries eventually
fill with sediment due to the natural processes of erosion and
sedimentation. However, the concern with Morro Bay is that these
natural processes have been accelerated due to anthropogenic
watershed disturbances. Studies conducted by various authors over
the past 25 years have concluded that the rate of sedimentation to
Morro Bay has rapidly increased. These studies have provided either
estimates of sediment loadings to the Bay from the creeks emptying
into the Bay, or estimates of sediment accumulations within the
Bay.
2.2 Water Quality Standards Water quality standards as set forth
in the Central Coast Region’s Water Quality Control Plan (Basin
Plan) include the identified beneficial uses of a waterbody, the
water quality objectives for those uses, and the antidegradation
policy of the State Water Resources Control Board.
2.2.1 Beneficial Uses The listed beneficial uses for the
waterbodies in the Morro Bay watershed are shown in Table 1 and
described below.
Table 1. Identified Uses of Inland Surface and Coastal Waters of
the Morro Bay Watershed.
Waterbody Name MAR
NAV
MUN
AGR
I ND
GWR
REC 1
REC 2
WI LD
COLD
WARM
MI GR
S PWN
B I OL
RARE
E S T
FRSH
COMM
AQUA
SHELL
Inland Surface Waters
Morro Bay Estuary ● ● ● ● ● ● ● ● ● ● ● ● ● Chorro Creek ● ● ● ●
● ● ● ● ● ● ● ● ● ● Dairy Creek ● ● ● ● ● ● ● ● ● ● ● San Luisito
Creek ● ● ● ● ● ● ● ● ● ● ● San Bernardo Creek ● ● ● ● ● ● ● ● ● ●
● Los Osos Creek ● ● ● ● ● ● ● ● ● ● ● ● ● Warden Lake Wetland
● ● ● ● ● ● ● ● ●
Coastal Waters Morro Bay ● ● ● ● ● ● ● ● ●Source: Central Coast
Regional Water Quality Control Board Basin Plan. Marine Habitat
(MAR): Uses of water that support marine ecosystems. Navigation
(NAV): Uses of water for shipping, travel, or other transportation
by private, military, or commercial vessels. Municipal and Domestic
Supply (MUN): Uses of water for community, military, or individual
water supply systems including, but not limited to drinking
water.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
3
Agricultural Supply (AGR): Uses of water for farming,
horticulture, or ranching. Ground Water Recharge (GWR): Uses of
water for natural or artificial recharge of ground water for
purposes of future extraction, maintenance of water quality, or
halting of saltwater intrusion into freshwater aquifers. Wildlife
Habitat (WILD): Uses of water that support terrestrial ecosystems.
Migration of Aquatic Organisms (MIGR): Uses of water that support
habitats necessary for migration or other temporary activities by
aquatic organisms. Preservation of Biological Habitats of Special
Significance (BIOL): Uses of water that support designated areas of
habitats, such as established refuges, parks, sanctuaries,
ecological reserves, or Areas of Special Biological Significance
(ASBS). Freshwater Replenishment (FRSH): Uses of water for natural
or artificial maintenance of surface water quantity or quality
which includes a water body that supplies water to a different type
of water body. Commercial and Sport Fishing (COMM): Uses of water
for commercial or recreational collection of fish, shellfish, or
other organisms. Aquaculture (AQUA): Uses of water for aquaculture
or mariculture operations. Industrial (IND): Uses of water for
industrial activities that do not depend primarily on water
quality. Water Contact Recreation (REC1): Uses of water for
recreational activity involving body contact with water, where
ingestion of water is reasonably possible. Non-Contact Water
Recreation (REC2): Uses of water for recreation activities
involving proximity to water, but not normally involving bodily
contact with water, where ingestion of water is reasonably
possible. Cold Fresh Water Habitat (COLD): Uses of water that
support cold water ecosystems. Warm Fresh Water Habitat (WARM):
Uses of water that support warm water ecosystems. Spawning,
Reproduction, and/or Early Development (SPWN): Uses of water that
support high quality aquatic habitats suitable for reproduction and
early development of fish. Rare, Threatened, or Endangered Species
(RARE): Uses of water that support habitat necessary, at least in
part, for the survival and successful maintenance of plant or
animal species established under state or federal law as rare,
threatened, or endangered. Estuarine Habitat (EST): Uses of water
that support estuarine ecosystems. Shellfish Harvesting (SHELL):
Uses of water that support habitats suitable for the collection of
filter feeding shellfish for human consumption, commercial, or
sport purposes.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
4
2.2.2 Water Quality Objectives The specific water quality
objectives that apply wholly, or in part, to sediment are contained
within the Central Coast Region’s Water Quality Control Plan (1994,
p. III-3) and are listed below: Settleable solids: Waters shall not
contain settleable material in concentrations that result in
deposition of material that causes nuisance or adversely affects
beneficial uses. Sediment: The suspended sediment load and
suspended sediment discharge rate of surface waters shall
not be altered in such a manner as to cause nuisance or
adversely affect beneficial uses. Turbidity: Waters shall be free
of changes in turbidity that cause nuisance or adversely affect
beneficial
uses. Increase in turbidity attributable to controllable water
quality factors shall not exceed the following limits: 1. Where
natural turbidity is between 0 and 50 Jackson Turbidity Units
(JTU), increases shall not exceed 20 percent. 2. Where natural
turbidity is between 50 and 100 JTU, increases shall not exceed 10
JTU. 3. Where natural turbidity is greater than 100 JTU, increases
shall not exceed 10 percent. Allowable zones of dilution within
which higher concentrations will be tolerated will be defined for
each discharge in discharge permits.
2.3 Description of Morro Bay and the Morro Bay Watershed Morro
Bay is a natural embayment located on the central coast of
California about 60 miles north of Point Conception and about 100
miles south of Monterey Bay (Figure 1). The Bay is situated
approximately in the middle of Estero Bay in San Luis Obispo County
(MBNEP, 2000. p. 2-1 draft). The Estuary is a shallow lagoon,
approximately four miles long and 1.75 miles at its maximum width
(Haltiner, 1988, p. 10). The water surface of the Bay is 523 acres
at Mean Low Low Water (Tetra Tech, 1999b, p. B-14). It was formed
in the last 10,000 to 15,000 years by the submergence of the river
mouth at the confluence of Chorro and Los Osos Creeks, the two main
drainages in the watershed. This submergence was a result of the
post-glacial rise in sea level of several hundred feet. Littoral
transport created the protective barrier beach (the sandspit) to
the west. Under natural conditions, two narrow entrances to the Bay
existed on either side of Morro Rock. The north entrance was
artificially closed in the early 1900’s, as discussed further under
tidal circulation and sediment flushing. The contributing watershed
area for Morro Bay is estimated to be 48,450 acres (USDA, SCS,
1989a). Chorro Creek drains 65 percent of the watershed and Los
Osos Creek drains the remaining 35 percent. The watershed’s highest
elevation is 2,763 feet above sea level and its farthest point from
the Bay is approximately 10 miles. The primary land uses are
agriculture, urban lands, and multi-use public lands (MBNEP, 2000,
pp. 2-11 draft). The geology of the watershed is a mix of igneous,
metamorphic and sedimentary rock less than 200 million years old.
Debris landslides, soil creep, and large slumps occur within this
terrain, usually triggered by intense rainstorms (USDA, SCS, 1989,
p. 2).
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
5
Torro Cree
k
Morro
Creek
Villa
Cre
ek
Chorro Creek
Islay CreekCoon Creek
Perry Creek
Los Osos Creek
Litt le
Mor
ro C
reek
San Luis
ito Creek
Cayucos C reek
Diablo Can
yon Creek
Old
Cree
k
ESTERO BAY
Warden Creek
Dairy
Cre
ek
Penn
ington
Cree
k
Clark Canyon Creek
Morro Bay Estuary
Point Estero
Point Buchon
S
N
EW
Figure 1.1 Location of MBNEP Study Area
MBNEP Characterization 1999
Estero Bay WatershedMorro Bay WatershedCreeks
Legend
Figure 1: The Setting of Morro Bay Source: MBNEP, 2000a. Figure
1.1, p.1-4
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
6
2.4 Stream Discharge into Morro Bay Stream discharge into Morro
Bay reflects the seasonal and annual variability in rainfall and
runoff characteristic of the Central Coast Region. Based on limited
stream gage data and rain gage data, discharge estimates were
developed from a hydrologic model completed by Tetra Tech (Table
2). At the mouth of Chorro Creek, the larger contributor of flow to
Morro Bay, average flows range from 1,476 cfs for a 2-year event to
35,390 cfs for a 100-year event. Farther upstream at Highway 1,
peak discharges can be several orders of magnitude above the
average for each event—an important factor in mobilizing and
delivering sediment to Morro Bay. Similar relationships hold for
Los Osos Creek.
Table 2. Estimated Discharge for Points along Chorro and Los
Osos Creeks for Events of Different Magnitudes.
2-year event Discharge,
cfs
5-year event Discharge,
cfs
10-year event
Discharge, cfs
25-year event Discharge, cfs
50-year event Discharge,
cfs
100-year event
Discharge, cfs
Basin Peak Avg. Peak Avg. Peak Avg. Peak Avg. Peak Avg. Peak
Avg. Chorro Ck
At Highway 1 52 15 340 77 779 162 1,763 349 2,865 521 4,341
773
At Mouth (below Twin
Bridge)
1,476 4,588 8,640 16,669 25,210 35,390
Los Osos Ck. At Upstream
Gage 34 9 237 42 603 91 1,479 203 2,420 307 3,625 462
At Mouth (below Warden
Ck.)
84 566 1,374 3,245 5,299 7,994
Source: Tetra Tech, 1998a, Table 4, p. 8, Table 8, p. 16, Table
11, p. 18.
2.5 Sedimentation in Morro Bay
2.5.1 Background Erosion Background erosion is considered to be
erosion that occurs in the absence of human influence on the
ecosystem. Disturbance in the drainage area has been significant
and the Soil Conservation Service conservatively estimates that
half of the erosion in the watershed is accelerated erosion (USDA,
SCS, 1989b. p. 31). The first significant land use change that
occurred in the watershed was the introduction of domestic grazing
animals (USDA, SCS, 1989a, p. 8). During the 1800s, a drought and
associated land use changes shaped the ownership patterns that
still exist today. Dairies and crops were established along the
valley floor, and creeks were rerouted to allow for roads,
residences, and crop production. The SCS indicates that
agricultural practices and estimated soil erosion rates in the
watershed are comparable to other areas in California. However, the
relatively shallow initial conditions of Morro Bay
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
7
and the fact that its configuration makes it an effective
sediment trap indicates that an effective upstream sediment control
program is required to prolong the life of the Bay (Haltiner, 1988,
p. 9).
2.5.2 Estimates of Sediment Loading The loss of Bay volume has
been caused primarily by creek-born sediment transport (Haltiner,
1989, p. 6). Sedimentation at the harbor entrance is dominated by
ocean transport, or longshore transport, whereas sedimentation in
the southern and eastern Bay is dominated by fluvial or river
transport. Wind is also a factor, as a good deal of sand is
naturally deposited within the estuary as winds blow across the
sandspit. From 1935 to 1987, the spit migrated 90 feet landward,
translating into a 30-acre loss of Bay mudflats (Josselyn, 1989, p.
7). Due to major changes in land use in the 1800’s, the rate of
sediment delivery to Morro Bay between 1890 and 1935 was estimated
to have been as much as 57,033 tons per year. Between 1935 and
1986, the rate decreased to an estimated 46,894 tons per year, due
to improved land use practices, agricultural methods, and the creek
system regaining balance after changes in the watershed (USDA, SCS,
1989a, p. 9). In 1998, Tetra Tech estimated that the average annual
sediment load to the Bay is 70,246 tons per year (1998a, p. 25).
This estimate is one and a half times greater than that estimated
by SCS in 1989, in part because the SCS study area excluded the
headwaters of Chorro Creek and its tributaries. Ten percent of this
total loading is sand and gravel, and 90 percent is fine material
such as clay and silt particles (Ibid.). (See Source Analysis for
more information on development of these estimates.)
2.5.3 Bathymetry and Sediment Flushing The ultimate fate of
sediment delivered to Morro Bay depends on the circulation and
flushing that occur there. Sediment fate is influenced by two
mechanisms, tides and freshwater inflows. The primary mechanism is
tidal exchange with the Pacific Ocean through the open entrance to
Morro Bay. The contours of the Bay bottom—its bathymetry—are an
expression of these mechanisms’ capacity to move sediment out of
the Bay and into the Pacific Ocean. Measurements of bathymetry
combined with total water area, permit calculations of total Bay
volume at varying depths, and of tidal prism. These are discussed
below to demonstrate the observed trend of increasing sedimentation
in Morro Bay. According to Haltiner, Morro Bay has lost 25 percent
of its total volume in the last 100 years, with some areas showing
greater decreases (1998, p. 6). Haltiner estimated that under
"normal" circumstances, the Bay would naturally fill in with
sediment in several thousand years but, if the present accelerated
rates continue, open water areas would fill in within the next 300
years (Ibid., p. 45). In 1998, Tetra Tech conducted a bathymetric
survey and developed a Tidal Circulation model for the MBNEP. The
general bathymetry of the Bay consists of extensive areas of
mudflats with little variation in slope, and steep-sided channels
+that cut through the mudflats (Figure 2). The depth and width of
these channels show considerable variability. Tables 3 and 4
include historic acreage and volume at various depths in the
Bay.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
8
Cho
rro C
reek
Los Osos Creek
Pacific Ocean
City of Morro BayCity of Morro Bay
1 foot
4 feet
Morro Bay W atershed Boundary
3 feet
2 feet
Mean low tide
Los OsosLos Osos
Southern T-Pier
S
N
EWFigure 2.3 Map Showing Current Bathymetry of Morro Bay
MBNEP Characterization 1999Source: TetraTech Bathymetry Survey
1999
Legend
CreeksLow salt marshHigh salt marshBelow mean low tideMean low
tide
Above mean low tide(1 foot intervals)
Figure 2: Map Showing Current Bathymetry of Morro Bay Source:
Tetra Tech Bathymetry Survey 1999 as presented in MBNEP, 2000.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
9
Table 3. Hyposometric (Area vs. Depth) Data Summary for Morro
Bay, 1884 to 1998.
Height above Mean Low Low Water (MLLW) (feet)
1998
1987
1935
1919
1884
Cumulative Area (acres)
Change in
Area 1884-1998
5 High Tide (MHHW) 2,024 4 Mean High Water (MHW) 1,897 1,891 3
1,697 1,805 2,158 2,155 2,240 -15% 2 1,475 1,521 2,001 1,900 2,110
1 Mean Low Tide (MLW) 1,147 1,155 1,733 1,743 1,985 -26% 0 Low Tide
(MLLW) 523 629 1,423 1,455 1,697 -32% -1 Extreme Low Tide 388 361
907 1,047 1,255 -58% -2 358 315 673 780 955 -59% -3 336 287 267 350
592 -4 318 221 249 255 -5 301
Source: Adapted from: Table B-2, Tetra Tech, 1999b.
Table 4. Adjusted Volume-Depth Relationship for Morro Bay,
1884-1998.
Height above Mean Low Low Water (MLLW) (feet)
1998 1987 1935 1919 1884 Changes in Volume
Cumulative Adjusted Volume (acre-feet)
5 Mean High High Water (MHHW)
11,884
4 Mean High Water (MHW)
9,923 9,316 10,516 11,216 12,216 -19%
3 8,126 7,616 8,516 9,316 10,216 -20% 2 6,540 6,116 6,716 7,416
8,116 -19% 1 Mean Low Tide (MLW) 5,229 5,016 5,416 5,916 6,416 -18%
0 Low Tide (MLLW) 4,394 4,316 4,516 4,816 5,116 -14% -1 Extreme Low
Tide 3,939 3,916 3,816 4,116 4,116 -4% -2 3,566 3,566 3,416 3,666
3,516 1%
Adapted from Table B-4b, Tetra Tech, 1999b). By comparing the
1998 bathymetry survey data with the historical estimate of 1884
water depths, Tetra Tech determined the following: Area
The entire area of Morro Bay at high tide has decreased by
approximately 15 percent to about 2,024 acres.
The area covered by water at low tide has decreased by 60
percent to 523 acres in 1998. Volume
The volume of water in Morro Bay at MHW has decreased by
approximately 20 to 25 percent or 2,000 acre-feet.
The decrease of volume of water in Morro Bay at MLW is
approximately 18 to 22 percent or 1,200 acre-feet.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
10
Volume of water below –1 ft MLLW, which approximates the
smallest volume of water remaining in the Bay during an extreme low
tide, has remained relatively constant, decreasing by five percent,
which is probably less than the accuracy of the assumptions and
measurements used in the calculations (Tetra Tech, 1999b, p.
B-15).
These results imply that encroachment from the margins and
aggradation of the shallowest areas within the Bay are the
processes causing the decrease in volume (Tetra Tech, 1999b. p.
B-19). Tidal Prism The tidal prism is defined as the difference
between the mean high water volume and the mean low water volume in
an estuary. The volume of the tidal prism relative to the total
volume of the Bay influences the flushing characteristics, tidal
current speeds and the sediment transport and scouring
characteristic of tidal currents. The decrease of tidal prism
volume in Morro Bay between 1881 and 1998 is equivalent to a 20
percent to 30 percent reduction (Ibid.).
2.5.3.1 Flushing and Circulation Tetra Tech developed a model to
determine which areas of Morro Bay are susceptible to poor flushing
under different flow conditions in Chorro and Los Osos Creeks
(Tetra Tech, 1999b). The three stream flow rates included a
low-flow condition typical of summer, a medium-flow rate of 64 cfs
at Chorro Creeks and 3.3 cfs at Los Osos Creek, and an extreme
high-flow rate of 1,146 cfs at Chorro Creek and 203 cfs at Los Osos
Creek. For the low- and medium-flow conditions, the model predicted
that the least flushing occurs in the southwest portion of the Bay
and inside the State Park Marina with flushing half-life times
ranging from approximately 9 to 18 days. The high-flow simulation
indicated extremely fast flushing throughout the Bay with a maximum
half-life of seven days in the extreme southwest corner of the Bay.
The Bay-wide average flushing half-life times for the low-flow,
medium-flow, and high-flow conditions are 4.2 days, 3.2 days, and
1.1 days, respectively. The simulations developed by Tetra Tech
indicate that the freshwater flows from Chorro Creek and Los Osos
Creek have a significant effect on flushing in Morro Bay. During
the low-flow conditions that persist through summer, the Bay—in
particular the southwest portion—is susceptible to a build up of
pollutants, including sediment (Tetra Tech, 1999b, p. 5-2). Tidal
influence and effects from the Morro Bay Power Plant are localized
to the mouth of the Bay. Sediment has been observed to collect in
front of the Morro Bay Power Plant seawater intakes. Approximately
5,000 cubic yards of sediment is dredged from in front of the Morro
Bay Power Plant intake every five to ten years (Jay, 2000, p. 4).
The distinction between processes occurring in the interior and at
the mouth of the Bay are reflected in the type of material
accumulating, which—aside from aoelian input of sand—are primarily
silts and clays of fluvial origin, as opposed to the sand dominated
sediments found at the mouth of the Bay (Ibid.). Therefore, tidal
transport of sediment, human alterations at the mouth of the Bay,
and Morro Bay Power Plant intake influences are secondary relative
to the load from the creeks. It is likely that structural changes
to the mouth of the estuary, in addition to the dynamics of
outgoing tidal velocity and incoming sediment transport, have
altered the dynamics of sand dominated transport at the mouth of
the estuary. From 1941 to 1946 the Army Crops of Engineers dredged
the Bay to create navigation channels and constructed breakwaters,
a dike extending 1600 feet from Morro Rock to the main land, a
stone groin and a revetment. It has not been quantified how these
structural changes have specifically altered flushing dynamics in
the estuary, but the rate of dredging has increased from an
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
11
average of every five years between 1944-1975 to every two or
three years currently (Noda and Jen, 1975).
2.5.4 Sedimentation in Chorro and Los Osos Creek Watersheds
Several factors have affected the channels of Chorro and Los Osos
Creeks over the years, including accelerated erosion within the
watershed from land uses, replacement of Twin Bridges, building of
levees, and subsequent dredging. These alterations have affected
not only the downstream portions of the creeks, but also reaches
several miles above the mouth. Aggradation (an increase in
sedimentation resulting in raised streambed elevations) in the
lower reaches of Chorro and Los Osos creeks has reduced the
capacity of these creeks to transport coarse sediments. Rather than
only building outward into the Bay, the portion of the delta
adjacent to Chorro Creek is now building upward. Major portions of
the delta have been raised two to three feet as a result of channel
overtopping and sediment deposition during major floods. However,
with bayward expansion of the delta, Chorro and Los Osos Creek
channels have become longer with a flatter slope, which in turn
increases sedimentation in the channel because of decreased flow
velocities. There is evidence of up to seven feet of channel
aggradation in Chorro Creek, and similar amounts in Los Osos Creek
(Josselyn, Los Huertos, 1991, pp.7, 9). The changes due to
increased sedimentation are most evident in the delta formed by
Chorro and Los Osos Creeks and in the southern portion of the Bay
in general. Accumulated sediment has caused the creek bottom at
South Bay Boulevard to rise over 13 feet in the last 50 years
(Haltiner, 1989). Increased deposition of coarse sediment in the
vicinity of the crossing of South Bay Boulevard over Chorro Creek
required the replacement of Twin Bridges—a multimillion-dollar
undertaking. As part of the National Monitoring Program, Regional
Board staff and volunteers conducted quantitative analyses of
streambed sediments in the two major streams and in several of
their tributaries. While these data do not provide a baseline for
comparing numeric targets for fish gravel, they do provide a
description of surface particles in the creeks and point to
significant differences in the sediment regimes of the
subwatersheds. Regional Board staff analyzed surface particle sizes
in Chorro Creek and in Dairy Creek and Pennington Creek
subwatersheds collected between 1993 and 1997. In Chorro Creek just
downstream from the reservoir, the average dominant particle size
found in five transects was 33.5 mm. Average surface particle sizes
at Dairy Creek and Pennington Creek were 15.25 mm and 12.75 mm,
respectively. Data collected during the 1994 sampling period were
omitted from the analysis as they included values much lower than
in other years, possibly an outcome of the Highway 41 Fire
(CCRWQCB, 2002b). Regional Board staff also performed a qualitative
Habitat Assessment, which included an evaluation of bottom
substrate and embeddedness using California Department of Fish and
Games's Rapid Bioassessment protocols (1993, 1995, 1996). Results
from 1993-1999 assessments show that average scores in the upper
reaches of Chorro Creek, the Clark Canyon branch of Los Osos Creek,
and Dairy Creek were similar (Ibid.). Bottom substrate scores were
"sub-optimal" with scores between 10 and 20 percent fines.
Embeddedness was "sub-optimal", with gravel, cobble and boulder
particles between 25 and 50 percent surrounded by fine sediment
(particles less than 6.35 mm). Regional Board staff also found that
Pennington Creek had the best scores, with "optimal" embeddedness
of 0 to 25 percent. Average Pennington Creek bottom substrate
scores were between 10 and 20 percent fines. Regional Board staff
found that the Warden branch of Los Osos Creek and the lower
reaches of Chorro Creek (near Chorro Flats) were "marginal", with
average bottom substrate values between 20 and 50 percent fines and
embeddedness between 50 and 75 percent surrounded by fine sediment
(Ibid.).
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
12
2.5.5 Highway-41 Fire of 1994 The Highway 41 Fire began on
August 14, 1994 and burned a total of 16,850 acres in the
headwaters of all the major tributaries within the Chorro Creek
watershed (USDA, Forest Service, 1994, P.1). According to the SCS
(1989a), an even-aged plant community created an environment with a
high fire potential in the northern brushland portions of the
watershed. Table 5 shows total acres within each watershed and the
area burned. Heavy rains followed the fire, producing flooding in
the winter of 1994/95. Extremely high turbidity levels and
suspended sediment concentrations resulted from erosion in the
upper watershed (CCRWQCB, 1998, p. 30).
Table 5. Area Burned During Highway-41 Fire.
Watershed Total Acres Acres Burned % Burned San Bernardo Creek
5,424 3,920 72% San Luisito Creek 5,400 2,166 40% Pennington Creek
1,922 775 40% Dairy Creek 1,804 627 35% Upper Chorro Creek 2,300 36
2%
Total 16,850 7,524 45% Source: USDA, Forest Service, 1994,
p.1
2.6 Impacts to Beneficial Uses Excessive sedimentation in Chorro
Creek, Los Osos Creek, and the Morro Bay Estuary has impacted many
of the beneficial uses of these waterbodies. The following
describes the nature of the impairment to the extent it has been
documented.
2.6.1 Fish and Wildlife (RARE, MIGR, SPWN, WILD) Among the
numerous species of fish and wildlife that occur in the Morro Bay
Watershed, there are several endangered, threatened, or special
status species (Table 6).
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
13
Table 6. Special Status Species Dependent on Morro Bay Estuary
and Watershed
Species State Status Federal Status Brown pelican Endangered
Endangered California black rail Threatened California clapper rail
Endangered Threatened California red-legged frog Threatened
California sea-blite Endangered Chorro Creek bog thistle Endangered
Endangered Cuesta Grade checkerbloom Rare Indian Knob mountainbalm
Endangered Endangered Least Bell’s vireo Endangered Endangered
Morro Bay kangaroo rat Endangered Endangered Morro Manzanita
Threatened Morro Bay shoulderband snail Endangered Salt marsh
bird’s beak Endangered Endangered Southern sea otter Threatened
Southern steelhead trout Threatened Southwestern Willow Flycatcher
Endangered Swainson’s Hawk Threatened Tidewater goby Endangered
Western snowy plover Threatened
Source: MBNEP, 2000, pp. 3-25, 3-26, Table 3-4. The effects of
sedimentation on fish and wildlife typically derive from the
alteration of their habitat (further discussion of habitat
alterations follows this section). Indeed, the
sedimentation-induced shift in estuarine habitat from subtidal to
intertidal has most likely resulted in a change in the types of
fish and wildlife found in Morro Bay (Josselyn, et al, 1989, p.
15). However, attempting to relate species population trends to
sedimentation is difficult, since the lack of biological data on
Morro Bay during the period of most rapid sedimentation makes
historic comparisons impossible (Ibid., pp. 12, 21). Additionally,
it is difficult to isolate the effects of sedimentation from other
factors affecting species abundance and diversity, including the
effects of urban development, invasive species, and, perhaps most
importantly for aquatic species, freshwater diversion and pumping.
Nevertheless, the susceptibility of some species to the deleterious
effects of excessive sedimentation is known, and in certain cases
actual effects have been observed in Morro Bay and its
tributaries.
2.6.1.1 Fish The Tidewater goby (Eucyclogobius newberryi) is a
species of special concern in the State and a federally listed
endangered species. Its presence in Los Osos and Chorro Creeks was
recorded in 1970, 1976, 1981, and 1989. However, no tidewater
gobies were collected during a 1998 survey (Ibid., 1989, p. 11;
Tetra-Tech, 1999a, p. 4-17). These fish have a short life cycle
(usually one year) and specialized habitat requirements. In Morro
Bay their primary habitats are the creek mouths. However, siltation
occurs at these locations and silt has filled in pools and greatly
reduced aquatic habitat during low flow periods (Worcester, 1992,
p. 8.1-5). On August 18, 1997, the National Marine Fisheries
Service published a final rule listing the Central California Coast
and South/Central California Coast steelhead (Oncorhynchus mykiss)
Evolutionary Significant Units (ESUs) as threatened species under
the Endangered Species Act. While known to occur in Morro Bay,
particularly in Chorro and Los Osos Creeks, steelhead were not
collected in fish sampling
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
14
conducted in 1999, as care was taken to avoid sampling in areas
where this protected species was more likely to occur (e.g., shaded
pools) (Tetra-Tech 1999a, p. 4-17). The historical significance of
the Morro Bay Watershed as a steelhead fishery is shown through the
California Department of Fish and Game (DFG) habitat conditions
survey on Chorro Creek in 1976. DFG found that between Canet Road
and the Chorro Creek reservoir, the creek provided a significant
percentage of the summer nursery habitat for steelhead and
sustained about 60 percent of the juvenile steelhead populations
(Chappell, 1976). Morro Bay Estuary and Los Osos and Chorro Creeks’
ability to support fish populations is determined by habitat
availability and quality. Habitat availability is limited by
streamflow, stream gradients, and physical barriers. Habitat
quality is limited by channel bottom composition, pool structure,
water temperature, pH, dissolved oxygen, food supply, and
predation. However, this TMDL only addresses habitat quality
impacts associated with excessive sedimentation. The key habitat
problems in Morro Bay Estuary and Los Osos and Chorro Creeks
associated with sedimentation appear to be pool quality, gravel
quality (for spawning and food production), and changes in channel
structure. The discussion of these specific impacts of sediment
follows in section 6.6.2. Freshwater Habitat.
2.6.1.2 Reptiles and Amphibians Reptiles and amphibians have
been similarly affected by the sedimentation that has affected
fish. Red-legged frogs (Rana aurora draytoni) are known from at
least two locations on Chorro Creek and its tributaries. They are
found on the lower portions of watersheds, where lower creek
gradients produce slower, deeper flows. Quiet, moderately deep
pools with dense, overhanging vegetation is their ideal habitat.
Much of the lower watersheds of Los Osos and Chorro Creeks are
impacted by siltation, reducing the available habitat for
red-legged frogs (Worcester, 1992, p. 8.1-5). The western pond
turtle’s (Clemmys marmorata pallida) aquatic habitat requirements
are somewhat similar to that of the red-legged frog. Pond turtles
are found in permanent pool areas of Chorro Creek with abundant
underwater cover, including tangles of roots and submerged logs.
They require standing or slow-moving water that forms pools about
three feet deep and six feet in diameter with adequate bank cover.
A reduction in surface water elevation resulting from a decreased
flow rate will reduce the pools’ suitability (Marshall, 1995, pp.
3, 6).
2.6.1.3 Birds Coastal brackish marsh, a sensitive habitat
present at the mouths of the creeks, is being rapidly lost due to
sedimentation. This affects rare and/or endangered species such as
salt marsh bird’s peak, the California brackish water snail, and
the California black rail (MBNEP, 2000, p. 5-4).
2.6.2 Freshwater Habitat (COLD, WARM) Chorro and Los Osos Creeks
serve an important role as warm and cold freshwater habitat for the
spawning, reproduction, and early development of rare, threatened
or endangered species of aquatic organisms. Aquatic vegetation,
fish, and bottom dwelling organisms can be smothered by excessive
sedimentation, both in the estuary and in adjacent tributaries.
However quantitative data that document the level of impairment in
Morro Bay are limited.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
15
2.6.2.1 Riparian Habitat Riparian habitat exists in corridors
along creeks and includes tall overstory trees, shrubby vegetation,
and understory grasses and forbs. These areas provide nesting,
feeding and cover habitat for a number of birds, mammals, and other
species, and serve as wildlife corridors for migratory animals
(Ibid., p. 3-20). There are 147 acres of riparian habitat in the
lower 1-mile reach of both Chorro and Los Osos Creeks (Tetra Tech,
1999a, p. 5-19).
2.6.2.2 Elevated Turbidity and Suspended Sediment Elevated
turbidity and suspended solids can result in decreased light
penetration through the water column, impacting aquatic plants such
as eelgrass and the organisms dependent on them. Potential effects
on fish swimming directly in water in which solids are suspended,
include: alarm reaction, increased morbidity (reduced resistance to
disease, abrasion of gill tissue) and increased mortality.
Turbidity can also affect the efficiency of methods for catching
prey, reducing the catch per unit effort (Newcombe, 1997, p. 6). It
is possible to relate severity of ill effect to concentration of
suspended sediment and duration of exposure in: all life stages of
salmonids, adult estuarine and freshwater nonsalmonids, freshwater
invertebrates and freshwater flora (ibid. p. 8). However, data
describing these effects specifically in Morro Bay and its
tributaries are not available.
2.6.2.3 Fine Sediment in Spawning Gravels As described above,
sedimentation can affect the steelhead’s freshwater habitat and
interfere with the reproductive process when fine materials being
deposited smother the gravel beds that are critical for spawning.
Sediment can also fill the deep pools that smolts need to survive
dry periods. Eroding gravel banks provide a source of spawning
gravels for a stream, but erosion of fine-textured soils that
contain clays, silts, and fine sands, can reduce habitat quality
for fish. Steelhead use the Chorro Creek drainage as adult spawning
habitat and as nursery habitat for hatchlings and juveniles
maturing toward their seaward migration. During winter and spring
months when stream flows reach sufficient magnitude, steelhead
migrate from Morro Bay into Chorro Creek and its tributaries. They
require clean gravel substrate and clear swift-flowing waters for
spawning. They also require deep pools for the young fish to feed
and grow while protected from predators. Juveniles will remain in
these nursery areas for one or two years (Marshall, 1995, exhibit
95-4, pp. 2, 3). Sedimentation within streams fills deep pools on
which smolt depend during low flow periods (Josselyn, 1989, p. 11).
Regional Board staff found no spawning gravel surveys for Chorro
and Los Osos Creeks. Nevertheless, the excessive sedimentation
described by numerous authors suggests that many potential spawning
areas are buried by fine sediment (Josselyn, et al., 1989,
Marshall, 1995, Tetra-Tech, 1999a, Worcester, 1992). Fine sediment
in spawning gravels has several effects on fish survival,
including: 1) cementing them in place and reducing their viability
as spawning substrate, 2) reducing the oxygen available to fish
embryos, 3) reducing intragravel water velocities and the delivery
of nutrients to and waste material from the interior of the redd
(salmon nest), 4) and impairing the ability of young salmon to
emerge as free-swimming fish (Kondolf, 2000. p. 265, 266). This
statement relates to the SPAWN beneficial use and the potential for
settleable material to affect spawning redds. Increased suspended
sediment can also result in direct impacts to fish by clogging
their gills (Reiser, Bjornn, 1979). Visual observations on Chorro
Creek indicate that the upper reaches are 0-25 percent embedded in
fines, while smaller tributary streams are between 25-50 percent
surrounded by fine particles. No data has been collected for Los
Osos Creek.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
16
2.6.2.4 Lack of Suitable Pools for Rearing Habitat Pools in
Chorro Creek potentially suitable as rearing habitat are impacted
by fine and coarse sediment. Sedimentation in pools 1) reduces the
volume of available rearing habitat by filling in pools and burying
pool-forming structural elements such as large woody debris, 2)
reduces pool depth and therefore the cool water refuge associated
with temperature stratification, 3) reduces the availability of
fish cover as a result of decreased depths and the burial of large
woody debris and other structural elements, and 4) causes loss of
surface flow as pools are filled in resulting in less available
habitat and protection from predators. This statement relates to
the SPAWN and COLD beneficial uses and the potential for sediment
and settleable material to impact rearing habitat.
2.6.2.5 Channel Aggradation and Stream Channel Instability In
addition to these primary effects on steelhead and their habitat,
several secondary effects on freshwater habitat for other species
including western pond turtle, and red-legged frog have been
observed in Chorro and Los Osos Creeks. For example, observed
channel aggradation (Josselyn, et al., 1989, Worcester, 1992,
Marshall, 1995) results in the burial of large woody debris and
other structural elements, a loss of the stream's ability to
effectively sort gravel, and a potential reduction in the dominant
particle sizes. This statement relates to the COLD and EST
beneficial uses and the potential for sediment to impact stream
channel stability and habitat niches.
2.6.3 Estuarine and Marine Habitat (EST, MAR, BIOL) The
estuarine habitat of Morro Bay includes coastal wetlands such as
salt and brackish tidal marshes, and intertidal flats, as well as
deepwater channels, and coastal streams. This “estuarine system”
can be defined as consisting of deepwater tidal habitat and
adjacent tidal wetlands that are semi-enclosed by land but have
access to the open ocean, and in which ocean water is diluted by
freshwater runoff from the land (MBNEP, 2000. p. 3-1). Table 7
presents reported areal extent of the estuarine and riparian
habitats of Morro Bay. In addition to these dominant wetland types,
between 55 and 80 acres of brackish marsh and between 28 and 35
acres of freshwater marsh were identified in previous studies
(Josselyn et al, 1989, p. 7, and MBNEP, 2000, p. 3-19).
Table 7. Areal Extent of Estuarine Habitat in Acres Reported by
Various Investigators
Haydock 1960
Josselyn, et al 1989
Chesnut 1996
Chesnut 1999
Tetra Tech 1999
Chesnut 1999
Chesnut 2000
Sampling Period
June-August
September September Spring, 1997
June, 1998
September, 1998
November, 1999
Eelgrass 335 723 458 50 81 120 400
Mudflat 1,319
412
(Within State Park)
Salt Marsh
140 (Outside State
Park)
436
Source: Tetra-Tech 1999a, Table 6-1. p. 6-2; except: Chesnut
1999, and Chesnut, 2000, Josselyn, p.18.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
17
The observed larger trend in Morro Bay is a
sedimentation-induced shift in estuarine habitat from subtidal to
intertidal, expressed as an increasing area of salt marsh, and
decreasing deeper water areas supporting eelgrass (Josselyn, et al,
1989, p. 15). Additionally, riparian areas at the mouths of Chorro
and Los Osos Creeks may be increasing (MBNEP, 2000, p. 5-3). At the
same time, more localized alterations to habitat are evident as
well. For example, the Chorro Creek delta salt marsh has
experienced an invasion of brackish and freshwater exotic species
along the Creek’s natural levees. Shoaling and net increases in
sediment alter substrate elevations and water levels, which
significantly affect the extent of any single wetland type. For
example, as the potential growth area for eelgrass at or near Mean
Low Low Water experiences increased shoaling, its potential habitat
decreases (MBNEP, 2000, p. 3-14).
2.6.3.1 Loss of Eelgrass Habitat Dense stands of eelgrass
(Zostera) form meadow-like beds in the lower intertidal zone of the
Morro Bay Estuary. Eelgrass is a perennial, submersed marine
aquatic plant that usually grows from rhizomes, or root shoots.
Eelgrass beds serve as spawning and nursery grounds for many
species in the estuary and marine environment. The eelgrass beds in
Morro Bay are known as the largest and least impacted of any in
Central or Southern California (Chesnut 1999). They are the most
significant of their kind available to wintering populations of the
Black brant (Branta bernicla nigricans) in central and southern
California. The density and diversity of benthic fauna are several
times greater within the eelgrass beds than in other Morro Bay
habitats (MBNEP, 2000. p. 3-7). Estimates of eelgrass populations
(or habitat range) in the Bay have fluctuated widely. Some
fluctuations are due to natural variability, however, impacts to
this habitat from sediment have also been evident. Prior to 1997,
published estimates of eelgrass habitat ranged from 335 to 732
acres. Then, in the spring of 1997, eelgrass distribution was found
to be as low as 50 acres (Chesnut, 1999, p. 1). This well
documented decline coincides with the winter following the
destructive Highway 41 fire in 1994, and the concurrent end of the
1990's drought cycle (Ibid.). Tetra Tech identified 81 acres of
eelgrass in Morro Bay, but some “sparse” beds as defined by other
researchers were not included in that analysis (Tetra Tech, 1999a,
p. 6-2). In addition, the timing of the surveys (spring) was not
optimal for the eelgrass resource. Chesnut mapped about 120 acres
in September of 1998 (1999, p. 20). By November of 1999, the
resource had recovered to its more typical acreage, as evidenced by
sampling and maps prepared by Chesnut (Ibid.). About 400 acres of
eelgrass were documented in that report. In addition to the effects
of shoaling and increased sedimentation on substrate depths,
suspended and resuspended fine sediments and resultant reduced
water clarity may also affect the distribution and extent of
eelgrass beds in Morro Bay (Tetra Tech 1999a, p. 6-7). Increased
turbidity from sediment loads combined with excess nitrogen and
phosphorous may result in unbalanced algal growth that clouds the
water.
2.6.4 Summary of Biological Beneficial Use Impacts Sedimentation
is not the only stressor affecting the biological integrity of
Morro Bay. However, the effects of sedimentation are the subject of
this TMDL and can be summarized as principally affecting habitat
quantity and quality. Table 8 describes the impacts to habitat in
qualitative terms.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
18
Table 8. Summary of impacts to habitats associated with
sedimentation in Morro Bay.
HABITAT IMPACT Saltmarsh Expansion of salt marsh. Siltation in
upper reaches. Mudflat Loss of mudflat due to salt marsh expansion.
Increased mudflat area in south bay. Reduction in tidal prism.
Eelgrass Siltation of eelgrass and reduction in potential area for
colonization
by eelgrass. Subtidal Soft Bottom Siltation of channels.
Riparian Siltation within riparian habitat. Invasion of exotic and
upland species. Reduction of flood plain. Loss of anadromous fish
habitat. Source: adapted from Josselyn, et al., 1989, Table 1.
Josselyn described the impacts to biological resources in Morro Bay
as follows (1989, pp.30-31):
1. Degradation of stream bottom and brackish marsh habitat due
to sedimentation from Chorro and Los Osos Creeks.
2. Invasion by undesirable exotics within the riparian zone due
to an increase in elevation and frequency of disturbance.
3. Loss of steelhead and tidewater goby habitat within the upper
tidal limits of Chorro Creek due to filling of deep-water pools by
sediment. Decline in summer stream flows also contributes to
habitat degradation for these species.
4. Historic loss of the potential area that could support
eelgrass. 5. Decline in some species (i.e., Brant) dependent on
eelgrass beds, though this conclusion is
compounded by influences outside Morro Bay. 6. Greatest historic
reduction of acreage is at elevations, which support mudflats and
eelgrass
beds near the MLLW datum. This is the area in which many of the
Bay’s fish and wildlife resources either forage or find suitable
habitat.
7. Any declines in eelgrass beds in the future will likely lead
to the decrease in fish and waterfowl utilization of Morro Bay
especially in the case of catastrophic sedimentation events.
2.6.5 Water Contact and Non-Contact Recreation, Navigation
(REC1, REC2, NAV) The Bay is an important recreational area.
Sedimentation has impacted recreational activities such as
kayaking, boating, and wind surfing in that the area and volumes of
water in the Bay available for these activities have decreased.
Furthermore, the area of the mudflats exposed during periods of low
tide has further limited navigation during lower tides.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
19
2.6.6 Shellfish Harvesting, Aquaculture, and Commercial and
Sport Fishing (SHELL, AQUA, COMM)
One local oyster grower reported $30,000 in lost revenue
following the Highway 41 fire and winter storms (Williams Shellfish
Farms, 1998). While the mechanism by which sedimentation can affect
shellfish deleteriously is understood, beyond such reports, there
has been no documentation of these effects in Morro Bay. Most
commercial fishing in this area is conducted outside the Morro Bay
Estuary and there has been no documentation of the affects of
sedimentation in the Estuary on commercial fishing.
2.6.7 Industrial (IND) The Morro Bay Power Plant is located on
the north end of Morro Bay and is operated by Duke Energy of
Charlotte, North Carolina. The power plant’s interaction with the
Estuary is primarily through its use of seawater. The plant’s
boilers use natural gas to create steam to drive turbines that in
turn drive electrical generators. The plant pumps seawater (limited
to 725 MGD) from its intake structure located near the northernmost
end of Morro Bay. The seawater passes through the condensers and is
discharged into Estero Bay via tunnels and a canal at the base of
Morro Rock. The Regional Board through a National Pollution
Discharge Elimination System (NPDES) permit (MBNEP, 2000, pp. 2-12,
2-13) governs the plant’s discharge, and its use of seawater. The
plant has experienced direct impacts due to the high turbidity in
the Bay primarily from sediment suspended during dredging
operations. During periods of elevated turbidity, the intake water
plugs the seawater/heat exchangers resulting in costly repairs and
maintenance (Lott, 2000).
2.6.8 Municipal, Agricultural Supply, Freshwater Replenishment
(MUN, AGR, FRESH) These beneficial uses of the Morro Bay Estuary
and Chorro and Los Osos Creeks are not currently affected by
sedimentation.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
20
3. Source Analysis
3.1 General Overview The purpose of this source analysis is to
characterize the types, magnitudes and locations of sources of
sediment loading to Morro Bay and to Chorro and Los Osos Creeks.
Sediment sources are discussed in terms of the quantities they
generate, the types of erosion causing them, and the types of land
use from which they derive. A discussion of the methods (Section
3.2) by which these loads were calculated precedes the presentation
of quantities contributed by source (Section 3.3). This source
analysis only considers sediment delivered to listed waterbodies
through fluvial transport from erosion sources. Other sources,
including sand blown in from the barrier beach west of the Bay, and
ocean sediments carried into the Bay by tidal currents, are not
considered. Ocean sedimentation is not caused by anthropogenic
activities that can be controlled by the TMDL’s Implementation
Plan. Barrier beach sands are deposited into Morro Bay through
natural aeolian transport. While researchers believe this process
is accelerated by anthropogenic disturbance of dune vegetation, the
effects thus far appear to be minor in comparison with the natural
process (Haltiner, 1988, p. 74). For this reason, the source
analysis includes no estimates of the contribution of barrier beach
sand to deposition in Morro Bay.
3.2 Methods This discussion describes the methods used to
calculate 1) quantities of sediment produced annually by the
subwatersheds of Morro Bay, 2) quantities from certain types of
erosion in the subwatersheds, and 3) quantities derived from sheet
and rill erosion—the dominant erosion type—in different land uses.
While intermediate calculations are presented in this section, the
end results of these methods are discussed below in Section C.
Relative Contributions.
3.2.1 Base Load Estimation Methods This section describes the
several steps required to calculate base loads. Tetra Tech
delineated subwatersheds, generated flow statistics, and
constructed a sediment yield model based on the Universal Soil Loss
Equation. Characteristics of the watershed important in sediment
yield calculations include soil erodibility, the size and
classification of material in the top layers of soil, the
vegetative cover, land use practices, the slope and typical length
of overland flow of rainfall runoff, and the local runoff. For the
base load estimates developed by Tetra Tech for the Morro Bay
Watershed, this information was obtained from available maps and
from an available rainfall-runoff model. The information was
combined with measured flow and sediment concentration data to
calibrate a sediment yield model for two subwatersheds in the
Chorro Creek Watershed. These two subwatersheds, Walters Creek and
Chumash Creek, are gaged as part of an ongoing paired watershed
study, being conducted by Cal Poly and the Regional Board. The
results of the calibrated model were then extrapolated to remaining
portions of the Chorro Creek and Los Osos Creek watersheds taking
into account variations in local soils, topographic and hydrologic
factors as well as sediment trapping characteristics of Chorro
Reservoir. Tetra Tech then validated this procedure by comparing
expected sediment concentrations with measured sediment
concentrations at the mouth of Chorro Creek during the 1997 water
year (1998a. p. 5). The results of the model are expressed in tons
of sediment per storm event, for storm events that could occur at
frequencies of 2, 5, 10, 25, 50, and 100 years. Calculating a
weighted average of these quantities
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
21
allows the results to be expressed in tons/year—units that are
the basis of total maximum annual loads. A more detailed
description of the models follows.
3.2.1.1 Subwatershed Delineation Load estimates were developed
for subwatersheds within the Los Osos and Chorro Creek watersheds.
These subwatersheds were identified using USGS quadrangle maps and
represent areas of common characteristics (overland slope, drainage
density). The entire Morro Bay Watershed was divided into 70
subbasins, including 54 in Chorro Creek and 16 in Los Osos Creek.
These were then grouped into the major tributaries identified below
(Table 9) (Tetra Tech, 1998b). Figure 3 illustrates the major
subwatersheds of the Morro Bay Watershed.
Figure 3: Subwatersheds of Morro Bay. Source: TetraTech, 1998a,
p. 3.
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
22
Table 9. Subwatersheds of Morro Bay Watershed.
Subwatershed Area (square miles)
Chorro Ck. at Res. 3.67 Dairy Creek 2.52
Pennington Creek 3.09 San Luisito Creek 8.28
San Bernardo Creek 8.49 Minor tributaries1 11.42
Chorro Creek 48.57
Los Osos Creek 7.57 Warden Creek 12.93
Los Osos Creek 7.57
Morro Bay Watershed 56.14 Source: Based on Tetra Tech, 1998b,
Table 1, p. 2. 1 Includes Subwatersheds: Walters Creek, Chumash
Creek and Chorro Creek at Highway 1.
3.2.1.2 Flow Hydrologic factors required to develop event-based
sediment yield estimates include the peak flow and total runoff
volume associated with each flood event. These factors drive the
sediment loading model. Because measured rainfall runoff data are
available for only a limited number of events, locations, and
timespans, these data were obtained from the hydrologic
rainfall-runoff model completed in 1998 by Tetra Tech for the Morro
Bay National Estuary Program (1998b, p. 7). That model used the
U.S. Army corps of Engineers HEC-1 Flood Hydrograph Package. The
model simulates the runoff response of Chorro and Los Osos Creek
for recorded or hypothetical storm events occurring within the
watershed. For a given storm event, the model allows peak
discharges and/or hydrographs to be generated in 70 subbasins
within the larger Morro Bay Watershed (Ibid.). The data base used
for development and calibration of the rainfall-runoff model,
included: U.S. Geological Survey topographic maps, soils
information from the Soil Survey for San Luis Obispo, historical
peak discharge data available at several locations throughout the
watershed and collected by San Luis Obispo County Engineering
Department, and the 5-minute rainfall and streamflow records
collected in 1995 and 1996 as part of the “paired watershed” study
by Morro Bay National Monitoring Program (Ibid., 1998a).
3.2.1.3 Sediment Yield
3.2.1.3.1 Modified Universal Soil Loss Equation (MUSLE) Sediment
yield, or sediment yield refers to the rate at which sediment
passes a particular point in the drainage system. It is usually
expressed as volume or weight per unit of area per unit of time
(Leopold and Dunne, 1978, p. 678). The Modified Universal Soil Loss
Equation (MUSLE) was developed in 1975 to calculate sediment yield
to a given point in a watershed for a given flood event. Tetra Tech
based their
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
23
estimates of sediment yield on a form of the equation adjusted
for conditions in Chorro and Los Osos Creeks:
T = a K LS (QpV)b
T = sediment yield in tons for a flood event. Qp = the peak flow
associated with the event. V = the runoff volume associated with
the event. K = soil erodibility. LS = watershed slope length. a =
the summation of several factors, including soils, basin topography
and land use factors. b = an exponent that varies with location.
This equation was calibrated using the data on flow and sediment
concentrations available from the Walters Creek/Chumash Creek
paired watershed study. The calibration resulted in values for “a”
and “b.” The equation was then applied to the gauged and ungauged
portions of the watershed using known basin characteristics (area,
erodibility (K), and watershed slope length (LS)) and hypothetical
hydrologic data (Q and V) from the rainfall runoff model. Event
total sediment yield tonnages were then calculated for each of the
major drainages and the sum of other minor tributaries in the
Chorro Creek and Los Osos Creek Watershed. In the case of Chorro
Creek the yield estimates were adjusted for trapping of coarse
sediment in Chorro Reservoir (Tetra Tech, 1998a. p. 10; Felhman,
2000).
3.2.1.3.2 Method for Calculating Average Annual Total Yields to
Bay from Chorro and Los Osos Creeks
Events for which sediment yields were developed, included the
2-year, 5-year, 10-year, 25-year, 50-year, and 100-year events.
These event-based yields then became the basis for determining
average annual yield. This required an assessment of the “average”
hydrology of a typical year, including an estimate of the
contribution from peak flood flows and low flows (Ibid., 1998a. p.
24). Regional Board staff applied the following formula to the
event-based loads developed for each Subwatershed: Average Annual
Yield = (100-yr yield*0.02)+(50-yr yield*0.01)+(25-yr
yield*0.04)+(10-yr yield*0.08)+(5-yr yield*0.2)+(2-yr yield*0.4)
Tetra Tech also developed independent estimates for total yield
from Chorro and Los Osos Creeks based on: 18 years of average daily
flow records at the Chorro Creek gage (at Canet Road), their
rainfall-runoff model, and the results of sediment yield and
transport calculations developed specifically for the Morro Bay
Watershed (Ibid., p. 15 –22). This served two purposes: first, this
method allowed them to partition the total load into its suspended
load and bed load fractions, and second, it provided a check on the
yields as calculated by the weighted average method applied to
develop subwatershed loads. In developing these independent
estimates, the following regression equations for sediment delivery
(both total load and the bed load fractions) were developed for
each creek:
Chorro Creek: Total Tons Chorro mouth = 0.005256 x (Avg Q Chorro
mouth)2 . 2 1 2 Bed Material Tons Chorro mouth = 0.0710 x (AvgQ
Chorro mouth)1 . 5 3 9 Los Osos Creek: Total Tons Los Osos mouth =
0.032981 x (Avg Q Los Osos mouth)2 . 1 1 8 Bed Material Tons Los
Osos mouth = 0.002784 x (Avg Q Los Osos mouth)1 . 9 0 1
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
24
Where Total Tons represents the net total delivery to the Bay
from the location indicated, and Bed Material Tons represents the
bed material delivery to the Bay from the location indicated, for
the given average daily flow value. These regression equations were
applied to each day in the selected 18-year hydrologic record, and
the summation of total tons delivered, divided by 18, provides an
approximation of the average annual yield (Ibid., pp. 24, 25). The
total loading calculated through this method was within one percent
of the loading as calculated through the weighted average
method.
3.2.2 Methods to Assign Loads to Erosion Types
3.2.2.1 Identify types, calculate rates, calculate percentages
(SCS) The Soil Conservation Service (SCS) identified principal land
use and vegetation types in Morro Bay based on 1978 data from the
Department of Water Resources Cropland Maps (USDA, SCS, 1989a. p
4). Rangeland, brushland, woodland, three types of croplands, and
urban lands were the major land use categories identified. The land
use patterns were then used to develop estimates of erosion by crop
or land use. Four source categories of erosion were identified by
SCS: sheet and rill2, streambanks, roads, and gullies3. The SCS
used the Universal Soil Loss Equation (USLE) to estimate erosion
from sheet and rill erosion—the dominant type of erosion. They used
the Direct Volume Method to calculate erosion from streambanks,
roads, and gullies. This method estimates the average annual
thickness of bank or surface removed by erosion; then multiplies it
by the area of bank or surface to give a volume estimate (USDA,
SCS, 1989a. p. 16). The sediment loads calculated by SCS from these
methods allowed for a breakdown of erosion types expressed as a
percentage of the total for Chorro and Los Osos Creeks (Table 10).
Sheet and rill erosion is the dominant source, accounting for
approximately 61-65 percent of total loading in the two
tributaries. In Chorro Creek sheet and rill erosion contribute
38,945 tons/year and in Los Osos Creek 5,935 tons/year.
Table 10. Erosion Categories and Percent Contribution in Morro
Bay Watershed.
Sheet and Rill Percent of
Total
StreambanksPercent of
Total
Roads Percent of
Total
Gullies Percent of
Total
Total Load Sheet and Rill Only (tons/yr)
Chorro Creek 64.9% 20.3% 13.9% 1.0%
19,200
Los Osos Creek 61.0% 21.4% 16.4% 1.3%
9,700
Watershed Total 63.5% 20.7% 14.7% 1.1%
28,900 SCSyield%fromC&L(tt)
2 Sheet Erosion: when rainfall intensity exceeds infiltration
capacity, the ground cannot absorb all the moisture, and water is
ponded on the
surface in small local depressions. These depressions eventually
are overtopped, and water runs off the surface in thin laminar
sheets. If the flow is sufficient to entrain soil particles, sheet
erosion occurs. Rill Erosion: Rills are small linear, rectangular
channels that cut into a slope surface. They tend to be parallel,
and they are most commonly observed on new road cuts (Chorley, et
al, p. 264). With continued sheet flow, a point is reached where
small rills appear, and flow becomes concentrated into larger
rills, which eventually become gullies.
3 Gully Erosion: Gullies are “arbitrarily defined as recently
extended drainage channels that transmit ephemeral flow, have steep
sides, and a steeply sloping or vertical head scarp…” (Selby, 1982,
p. 107). Because they are very rapidly developed erosional forms
they are usually not regarded as features of normal erosion, but
the result of changes in the environment, such as burning of
vegetation, overgrazing, climatic change affecting vegetation, and
extreme storms (ibid.).
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
25
Source: adapted from Table 3, USDA, SCS, 1989a. pp.16, 17.
3.2.3 Method to Assign Loads to Land Use Types
3.2.3.1 Determining Land Use on a Subwatershed Basis Regional
Board staff chose to perform subsequent analyses of sources based
only on the sheet and rill component, since it is the dominant
source throughout the watersheds. A Subwatershed basis for these
sources was needed to provide a better understanding of sources.
However, a subwatershed breakdown of landuses for the Morro Bay
Watershed was not available from the SCS documents that identified
the erosion types. Therefore, Regional Board staff used a
Geographic Information System (GIS) to calculate landuses acreages
within each subwatershed. Staff retained the five land use classes
(Rangeland, Brushland, Woodland, Cropland, and Urban) identified by
SCS. The GIS included layers from the UC Santa Barbara Geographic
Approach to Planning4 (GAP) to calculate subwatershed-based land
uses areas (Table 11). This required an aggregation of vegetation
and landuse types to conform to the SCS classification. Regional
Board staff relied upon the USDA Forest Service Wildlife-Habitat
Relationships (WHR) to aggregate the land uses for each of the
subwatersheds (CDF, 1988).
4 GAP data were obtained from the UCSB Gap Analysis FTP site
at:
ftp://lorax.geog.ucsb.edu/pub/data/gap_analysis/. (At time of
this writing, the site has been replaced by
http://www.biogeog.ucsb.edu/projects/gap/gap_home.html ). The data
files were downloaded for the Central Western Ecoregion on 6/12/97.
The ARC/INFO Export files were imported into MIPS, converted from
Albers to UTM projection and exported as Arc shapefiles. Accessory
tables were transformed directly from ARC/INFO to .dbf format using
ArcView's Export71 program. Key fields from these "lookup" tables
(primary and secondary species, Holland communities, and
Wildlife-Habitat Relationships (WHR) habitat types) were
incorporated into the shapefile attribute table by joining the
relevant lookup table to the attribute table and saving as a new
shapefile. The GAP data shows vegetation as interpreted and
classified from a 1990 Landsat satellite image. The best fields to
use for viewing the polygons are "Holland1Name" or "WHRType1".
-
ATTACHMENT B Draft Morro Bay Total Maximum Daily Load for
Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay
Estuary)
26
Table 11. Land Uses (acres) within Chorro and Los Osos Creek
Watersheds.
Land Uses Subwatersheds Rangeland Brushland Woodland Cropland
Urban Other Total
Chorro Creek at Res.